Polyimides find extensive use in electronic applications where they are useful in forming dielectric films on electrical and electronic devices such as capacitors and semiconductors. Typical uses for polyimides include protective coatings for semiconductors, dielectric layers for multilayer integrated circuits, high temperature solder masks, bonding multilayer circuits, final passivating coatings on electronic devices and the like.
It is well known in the polymer art to make thermally stable all-aromatic polyimides by the condensation polymerization of dianhydrides and diamines to form polyamic acid. Such polyimide precursors are disclosed inter alia in U.S. Pat. No. 3,179,634 to Edwards. Solutions of such polyamic acids in aprotic solvents are then coated on appropriate substrates and then dehydrated to the corresponding polyimides by heating at high temperatures, e.g., 300.degree. to 400.degree. C.
In each application the polyimide is applied as a liquid or in the form of its polyamic acid precursor which then must be cured by heating to form the polyimide. A typical high performance aromatic polyimide does not naturally adhere well to inorganic substrates, and areas having poor adhesion can become sites for later metal corrosion.
Heretofore, the problems of adhesion of polyimide or polyimide precursors to inorganic substrates were approached mainly by applying a layer of adhesion promoter to the substrate which was capable of serving as a reaction site with the polyamic acid precursor of the polyimide. However, this involves an additional step in the already complex fabrication process for electronic components.
To avoid such an extra processing step, there have been attempts to incorporate adhesion promoters directly into the polyimide or polyamic acid coating solution. Aminosilane materials such as N-(triethoxysilylpropyl)amino maleic acid have been used for this purpose. However, such one-step application systems frequently have the disadvantage that they require that the initial polymer cure be carried out at a temperature of 250.degree. to 300.degree. C. in air to obtain adequate adhesion. It would therefore be highly advantageous to have a one-step system which would not require a high temperature cure in air, but can be carried out in a non-oxidizing atmosphere such as nitrogen or argon to reduce device metal oxidation.